1. Field of the Invention
The present invention relates to an exchange coupled film which includes a seed layer, an antiferromagnetic layer, and a ferromagnetic layer deposited in that order from the bottom and in which the magnetization direction of the ferromagnetic layer is pinned in a predetermined direction by an exchange coupling magnetic field produced at the interface between the antiferromagnetic layer and the ferromagnetic layer, and to a magnetic sensing element, such as a spin-valve thin-film element or anisotropic magnetoresistive (AMR) element, using the exchange coupled film. More particularly, the invention relates to an exchange coupled film in which current-carrying reliability (electromigration resistance) and the rate of change in resistance can be appropriately improved even if the recording density is increased, and to a magnetic sensing element using such an exchange coupled film.
2. Description of the Related Art
FIG. 18 is a partial sectional view of a conventional spin-valve thin-film element, viewed from a surface facing a recording medium.
As shown in FIG. 18, an antiferromagnetic layer 30, a pinned magnetic layer 31, a nonmagnetic interlayer 32, a free magnetic layer 33, and a protective layer 7 are deposited in that order on a seed layer 14 which is, for example, composed of a NiFeCr alloy.
In such a spin-valve thin-film element, an exchange coupling magnetic field is produced at the interface between the antiferromagnetic layer 30 and the pinned magnetic layer 31 by annealing, and the magnetization of the pinned magnetic layer 31 is pinned in the height direction (in the Y direction in the drawing).
In the spin-valve thin-film element shown in FIG. 18, hard bias layers 5 are formed at both sides of a laminate including the seed layer 14 to the protective layer 7, and the magnetization of the free magnetic layer 33 is aligned in the track width direction (in the X direction in the drawing) by a longitudinal bias magnetic field from the hard bias layers 5.
As shown in FIG. 18, electrode layers 8 are disposed on the hard bias layers 5. Although a sensing current from one of the electrode layers 8 needs to flow through three layers, i.e., the pinned magnetic layer 31, the nonmagnetic interlayer 32, and the free magnetic layer 33, the sensing current is also shunted to the seed layer 14 and the antiferromagnetic layer 30 in this structure.
By providing the seed layer 14 under the antiferromagnetic layer 30, the {111} orientations of the individual layers formed on the seed layer 14 are improved, and the crystal grain size in the planar direction of the layers (in the X-Y planar direction) is considered to be increased, and therefore, an improvement in current-carrying reliability, for example, electromigration resistance, an improvement in the rate of change in resistance (ΔR/R), and an improvement in the soft magnetic properties of the free magnetic layer 33 are expected.
In order to improve the {111} orientations of the individual layers formed on the seed layer 14 and to increase the crystal grain size in the planar direction, the seed layer 14 must have a face-centered cubic structure (fcc structure) and the surface of the seed layer 14 must have satisfactory wettability. If the surface of the seed layer 14 has satisfactory wettability, when the antiferromagnetic layer 30 is deposited on the seed layer 14 by sputtering, the atoms of the antiferromagnetic material constituting the antiferromagnetic layer 30 do not easily aggregate, and the orientation in the planar direction of the antiferromagnetic layer 30 can be more strongly aligned to the {111} plane which is the closest-packed plane.
Although the higher Cr content in the seed layer 14 is considered to be preferable in order to improve the wettability, if the Cr content becomes excessive, a body-centered cubic structure (bcc structure) starts to appear in the crystal structure in addition to the face-centered cubic structure (fcc structure), and therefore, the {111} orientations of the individual layers on the seed layer 14 are degraded, resulting in a degradation in current-carrying reliability and a decrease in the rate of change in resistance.
In the known art, by setting the Cr content in the seed layer 14 at 35 atomic percent or less, or at 40 atomic percent or less, the crystal structure of the seed layer 14 is kept in the face-centered cubic structure.
However, as the recording density is increased, spin-valve thin-film elements are further miniaturized, and thereby the density of the sensing current flowing in the spin-valve thin-film elements is increased. Consequently, electromigration may occur, the rate of change in resistance may be decreased due to an increase in resistance, and noise may occur.
In order to overcome the problems described above, it is effective to improve the wettability of the surface of the seed layer 14 so that the {111} orientations of the individual layers on the seed layer 14 are further improved, and to increase the crystal grain size in the planar direction so that the electric conductivity is improved. For that purpose, the Cr content in the seed layer 14 must be increased more than which has been conventionally set. However, if the Cr content is set at 35 to 40 atomic percent or more, the body-centered cubic structure (bcc structure) appears in the crystal structure of the seed layer 14 in addition to the face-centered cubic structure.
If the body-centered cubic structure is mixed with the face-centered cubic structure in the seed layer 14, the {111} orientations of the individual layers deposited on the seed layer 14 cannot be improved, and the crystal grain size cannot be increased, resulting in a decrease in the electric conductivity. Consequently, it is not possible to produce a spin-valve thin-film element which is suitable for increasing the recording density using the conventional seed layer 14.